Elements of solid state devices are generally formed near the surface of a silicon substrate and covered with a layer of dielectric. Electrical connections to the elements are made by filling contact holes etched through the layer of dielectric with metal. Etching the contact holes by exposing the dielectric to appropriate gasses in a plasma through openings in a resist mask is sometimes uncontrollable because the carbon liberated from the resist mask forms a polymer in the contact holes that terminates the etching process before the contact holes reach a desired element.
In order to overcome this problem, a layer of polysilicon has been formed on the dielectric layer and made into a mask having apertures therein by exposing it to etching gasses through openings in a resist mask that is formed on top of it. The resist mask is removed and the contact holes are etched into the dielectric by exposing it to suitable ionized gasses through the apertures in the polysilicon mask. Because the resist mask is not present during the latter process, less carbon is produced and less polymer formed. Examples of this process are described in: U.S. Pat. No. 5,028,550 issued on Jul. 2, 1991 and entitled "Method of Manufacturing Semiconductor Device; U.S. Pat. No. 4,473,435 issued on Sep. 25, 1984 entitled "Plasma Etchant Mixture" and U.S. Pat. No. 5,279,989 issued on Jan. 18, 1994 and entitled "Method For Forming Miniature Contacts of Highly Integrated Semiconductor Devices."
As the dimensions of devices in a direction parallel to the substrate decrease, there is a need for the diameter of contact holes to be reduced. As noted in the U.S. Pat. No. 5,279,989, there is a limit to the minimum diameter of contact hole that can be attained using the resist masking process because the minimum diameter of the apertures formed in the polysilicon mask is too large. Although not stated in the patent, this diameter is 0.6 um when standard techniques are used. In order to effectively reduce the diameter of the apertures in the polysilicon mask in accordance with this patent, the resist mask used to form the apertures is removed and the new exposed polysilicon mask is coated with silicon dioxide. The silicon dioxide layer is then entirely removed except for the portion of it in contact with the inner edges of the apertures, which is referred to as a spacer, by a blanket etching process, but the polysilicon layer is not affected. Access holes are then etched in the dielectric by exposing it to etching gasses through the spacers that have a smaller diameter than the apertures in the polysilicon mask.